Typically, a seat of an automobile, airplane, or the like is provided with a seatbelt to protect a passenger in the event of an accidental collision, etc. To assure easy and simple attachment/detachment of such a seatbelt, the seatbelt is generally provided with a buckle. Conventionally, the seatbelt buckle consists of a lock mechanism having a lock plate, into which a tongue plate is inserted and locked, and a release mechanism having a release button to enable the tongue plate to be ejected out of the buckle.
In the conventional seatbelt buckle, to fasten the seatbelt around a passenger, a tongue plate, which is supported by the seatbelt, is inserted into the buckle in such a manner that the lock plate is inserted into a coupling hole of the tongue plate and simultaneously, an anti-release pin is located at a position to restrict upward movement of the lock plate. Then, to eject the tongue plate out of the buckle, the release button, which is used to release engagement between the tongue plate and the buckle, is pressed in a release direction, causing the anti-release pin to be moved to a non-coupling position. In a state wherein the tongue plate is completely inserted into a body of the buckle to thereby be locked in the buckle, in order to reliably maintain the locking of the tongue plate even if an external shock is applied to the buckle, a spring is provided to continuously press the lock plate to the locking position. The spring also serves to return the release button to an original position thereof. To facilitate easy engagement and disengagement between the tongue plate and the buckle, the release button is configured to release when only a slight force is applied thereto.
Recently, there have been proposed safety devices for preventing occurrence of several troubles, such as for example, a seatbelt pre-tensioner to prevent a seatbelt from being loosened from a passenger upon accidental collision of a vehicle, or a buckle pre-tensioner to pull down a buckle using instantaneous explosive power.
However, the pre-tensioner, which is proposed to prevent troubles caused by the loosened seatbelt, may apply instantaneous acceleration to the buckle during operation thereof, and thus, there is a risk that the locking of the tongue plate is unexpectedly released even though the release button is not pressed, causing ejection of the tongue plate out of the buckle. More specifically, if the buckle is instantaneously pulled to tension the tongue plate, or the tongue plate itself is pulled and tensioned, inertial force is applied to the release button or the anti-release pin in a release direction, causing the tongue plate to be forcibly released from the locked state thereof and be ejected out of the buckle.
In the case of the above-described conventional seatbelt buckle with no shockproof device, one might consider enhancing the elasticity of the spring used to press the release button, in order to prevent the unexpected ejection of the tongue plate. However, this requires an increase in the size of the spring, and consequently, an increase in a press force (i.e. release force) of the release button required to release the locking of the tongue plate against the spring, resulting in deterioration in safety.
For this reason, there have recently been proposed a variety of buckles with a shockproof device to effectively deal with inertial force of the buckle caused upon rapid acceleration. The shockproof device for a seatbelt buckle is configured in such a manner that an inertia lever is pivotally rotatably coupled to a body base inside the buckle so as to prevent unexpected movement of a release button in a release direction.
FIG. 1 illustrates one example of a conventional seatbelt buckle with a shockproof device, which is disclosed in German Patent Publication No. DE 9202526.9 U1. In the disclosed conventional seatbelt buckle with the shockproof device, regardless of movement of a release button in a release direction or a non-release direction, inertial force of the release button is applied to an inertia lever in a direction perpendicular to the movement direction of the release button. In the conventional shockproof device shown in FIG. 1, the inertia lever acts to remove the inertial force of the release button caused when the release button is moved in the release direction, thereby restricting the release movement of the release button. However, in order to reliably restrict the release movement of the release button, it is necessary to set an inertial force moment of the inertia lever higher than that of the release button.
In the above-described conventional shockproof device, under the assumption of setting a positive moment, if the release button is forced in a non-release direction, the release button is moved in the non-release direction by inertial force thereof. However, there is a risk that inertial force of the inertia lever, which comes into contact, at a cylindrical periphery thereof, with a straight vertical surface of the release button, is excessively larger than the inertial force of the release button that will be moved in the non-release direction, causing the release button to be unexpectedly moved in a release direction. Further, in the above-described shockproof device, although it is possible to set the same positive moment, this makes it difficult for the inertia lever to effectively deal with the inertial force of the release button with respect to the release direction or non-release direction, resulting in unreliable restriction in the release movement of the release button.
FIG. 2 illustrates another example of a conventional seatbelt buckle with a shockproof device, which is disclosed in Japanese Patent Publication No. 2005-0144138. The disclosed shockproof device includes means to generate a difference between a torque acting on an inertia lever by inertial force of a release button with respect to a release direction and a torque acting on the inertia lever by inertial force of the release button with respect to a non-release direction, so as to reliably maintain a tongue plate inside the buckle, regardless of the inertial force of the release button in any direction.
A problem of the conventional shockproof device shown in FIG. 1 is that, if the inertial force moment of the inertia lever is not equal to the inertial force moment of the release button, under the influence of inertial force of the release button not only in the release direction but also in the non-release direction, it is impossible to prevent the release button from being moved in a release direction using the inertia lever. Moreover, according to the direction of the inertial force, it may be difficult to reliably prevent disengagement between the tongue plate and the buckle.
A problem of the conventional shockproof device shown in FIG. 2 is that setting greater inertial force of the inertia lever than that of the release button to compensate for the inertial force of the release button so as to prevent disengagement between the tongue plate and the buckle requires an excessive increase in the mass and volume of the inertial lever.
In the above-described conventional seatbelt buckles using the inertia lever configured to be brought into contact with the release button, or the inertia lever configured to create inertial force moment sufficient to compensate for the inertial force of the release button, nonferrous metals or metal powders for sintering having a high specific gravity must be used due to a need to increase the mass and volume of the inertia lever. This inevitably results in increased material costs and high manufacturing costs depending on fabrication techniques. Furthermore, when the release button is pressed to release the seatbelt buckle, the heavy weight of the inertia lever may cause an excessive increase in disengagement force of the buckle. In addition, unnecessary operations of the inertia lever during general fastening/unfastening of the seatbelt buckle may cause failures in interconnections of components inside the buckle.